In the final image, the size of the Earth has been added to give a sense of scale. A rough comparison suggested that this prominence is approximately 7-Earth diameters high, reaching high into the sun's atmosphere (the corona).
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Prominences are large structures of magnetized plasma that can erupt from the sun's chromosphere and arc high into the corona. As the plasma in the sun's corona can have temperatures exceeding a million degrees Kelvin (Celsius), the plasma in prominences can be a hundred times cooler, at a similar temperature to the chromosphere.
As they are cooler than the surrounding corona, they generate visible light and can therefore be spotted fairly easily though specialized equipment such as solar telescopes and eclipse glasses. (NOTE: Never look directly at the sun as serious eye injuries and even blindness can result.)
Also, as the sun is rich in ionized hydrogen, astronomers use hydrogen alpha (or H-α) filters to produce observations of stunning detail of prominences and chromospheric features on the solar disk.
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